Known flow dryers commonly employ an inlet duct in series with an agitator followed by a drying duct. A stream of hot gas is blown through this series arrangement and matter entrained in the stream is dried and comminuted, a cyclonic separator is usually connected to the drying duct and is employed to separate the saturated gas from the dry comminuted product. Matter to be dried is usually fed into the agitator where it is broken up and partially dried and comminuted before it is transferred to the drying duct. The agitator presents resistance to flow through the flow dryer and this resistance depends on the nature and character of the matter being processed and how that matter interacts with the agitator to degrade the gas stream. Under these circumstances, the velocity of the air travelling through the drying duct and the actual drying conditions in the drying duct are unpredictable because the resistance in the agitator depends on a number of factors including the turbulence created by the agitator, the density of the air containing the partially comminuted matter in the agitator and the flow directional changes, of up to 280 degrees, consequent to the operation of the agitator on the partially comminuted matter.
The result of this is that there is difficulty in predicting the velocity of the gas through the drying duct and as a consequence, a difficulty in predicting the residence time for any particular material.
The applicant's U.S. Pat. No. 4,573,278, the disclosure of which is incorporated into the present specification, describes a flow dryer which incorporates a plurality of adjustable baffles in the drying duct in order to vary the path length of matter flowing through the drying duct, by virtue of this change in path length, residence time in the drying duct can be altered so the flow dryer can be adapted to process different materials having different moisture content. In some cases however, this arrangement does not provide a long enough residence time in order to provide adequate drying and it is usually not practical to extend the drying duct to give higher residence times. This is particularly the case where particles of comminuted matter, after having entered the drying duct reach their terminal velocity before drying is complete. The end result in this case is usually a wet product independent of the length of the drying duct.
While this proposal, under many circumstances enables different materials to be treated using the one machine the arrangement is not effective in some circumstances. In addition the arrangement of baffles is expensive to manufacture. It would therefore be desirable to provide an alternative means by which a flow dryer could be adapted to treat different materials.